Which HTML tag I should use to enclose such notes to add a semantic meaning of a note that may be useful to read at a given point of a tutorial, but is not part of the main tutorial flow?

A better description is in the specification itself. Why read secondary remarks when the source is written so good?

I respectfully disagree with your assessment. You are referencing the quote "It's not appropriate to use the aside element just for parentheticals, since those are part of the main flow of the document." However the OP specifically said that they are looking for a semantic element for "a note that may be useful to read at a given point of a tutorial, but is not part of the main tutorial flow". That is what "aside" is for. It's not part of the main content flow.

An admonition is a parenthetical

<aside> is appropriate if the side note "could be considered separate from the content"

<aside> is not appropriate if the side note is "a parenthetical". The W3C gives no examples of what it means.

In my opinion, the W3C definition is unnecessarily confusing and restrictive. The dictionary definition of aside is "a temporary departure from a main theme or topic", and the spec should just stick to that, rather than introducing subtle distinctions.

I believe the accepted answer is not quite correct. According to the HTML5 working draft, the <aside> element can be used to mark up side notes in certain, but not all cases:

Of course, there is no reason why you can't use <aside> for all sidenotes, if it makes your code simpler. Think of it as civil disobedience. :)

The dictionary definition of aside is "a temporary departure from a main theme or topic"

Oh, and from a language/design perspective, you can actually turn regular words in a sentence into channels, just as many people do with @replies. For example: I’m coming to #barcamp later today.

It also enforces actual use in the wild of tags, since no evidence of a tag will exist without it first being used in conversation. This means that representing channels in tagclouds across the site that grow and fade over time, and are contextual to all of Twitter or to a single user, is the ideal interface for displaying this information.

I also like that the folksonomic approach (as in, there are no “pre-established groups”) allows for a great deal of expression, of negotiation (I imagine that #barcamp will be a common tag between events, but that’s fine, since if there is a collision, say between two separate BarCamps on the same day, they’ll just have to socially engineer a solution and probably pick a new tag, like #barcampblock) and of decay (that is, over time, as tags are used less frequently, other people can reuse them — no domain squatting!).

Every time someone uses a channel tag to mark a status, not only do we know something specific about that status, but others can eavesdrop on the context of it and then join in the channel and contribute as well. Rather than trying to ping-pong discussion between one or more individuals with daisy-chained @replies, using a simple #reply means that people not in the @reply queue will be able to follow along, as people do with Flickr or Delicious tags. Furthermore, topics that enter into existing channels will become visible to those who have previously joined in the discussion. And, perhaps best of all, anyone can choose to leave or remove topics that don’t interest them.

This is how it works in IRC, and how it needed to work in Twitter.

Now, in thinking about implementing channels, it was imperative that I not introduce any significant changes into the way that I currently use Twitter any more than I have for other features that have been added to Twitter (for example, @replies or direct messages). Channels would need to be a command-line-friendly addition, and one that would require absolutely zero web-based management to make the most of it (to draw a distinction, Pownce fails this test with its Friend Sets, since it requires use of their website to take advantage of this feature).

Jaiku comes closest with their channels implementation, making it extremely easy to create new channels (simply post a message that begins with a hash (#) and your intended channel name — and if the channel doesn’t exist, it’ll be created for you):

I’m more interested in simply having a better eavesdropping experience on Twitter.

Encode the audio to a supported format, such as AAC by replacing -acodec copy with -c:a aac, or use a more flexible output container format that does support pcm_alaw, such as MOV or MKV.

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The downsides are that Firefox has slower startup times (because Chrome is constantly on in the background) and slower page loading speeds (because it doesn’t steal CPU cycles like Chrome).